The present invention relates to copolymers of vinyl esters, which are useful as binders in paint formulations.
Vinyl acetate polymers and copolymers are widely used as binders in architectural coatings formulations. These binders are relatively inexpensive, as compared to their acrylic counterparts, yet they have proven to be difficult to prepare as copolymers with pigment-adsorbing monomers such as phosphorus acid monomers or salts thereof. U.S. Pat. No. 6,710,161 teaches a distinct preference for copolymerizing a phosphorus acid monomer and a co-monomer at a pH of less than 2, and more preferably less than 1.5. The stated belief for the preference in carrying out the copolymerization reaction at a low pH (below the first pKa of the acid monomer) is that the phosphorus acid monomer is protonated and, therefore, less water soluble than it would be at higher pH; thus, polymerization of the protonated phosphorus acid monomer is believed to lead to increased incorporation of the monomer into the growing particle particles and a reduction of undesired water-soluble phosphorus acid polymer remaining in the reaction medium. Experimental results support this theory: A paint prepared using a poly-phosphoethyl methacrylate, methyl methacrylate, butyl acrylate binder prepared at pH 1.35 shows superior solvent resistance to a paint prepared using a binder made from the same monomer mix but at a pH of about 2.2.
The patent teaches, by inference, that adsorbing latexes of vinyl acetate and a phosphorus acid monomer should be exceedingly difficult to prepare: If polymerization of vinyl acetate and a phosphorus acid monomer were carried out at a pH above the first pKa of the phosphorus acid monomer (generally above 2), one would predict unacceptable levels of non-incorporated phosphorus acid oligomers or polymer in the aqueous phase; however, at low pH, vinyl acetate is known to hydrolyze.
U.S. Pat. No. 7,179,531 teaches a two-stage “acorn” copolymer comprising a crosslinked acrylic core containing structural units of phosphoethyl methacrylate, butyl acrylate, and methyl methacrylate protuberating from a (poly)vinyl acetate-butyl acrylate shell. Significantly, the core contains no structural units of vinyl acetate, which is known to hydrolyze under the low pH conditions of the first stage polymerization reaction. Therefore, no actual copolymers of phosphoethyl methacrylate and vinyl acetate were prepared in a single stage in this complex and costly 2-stage process.
It would therefore be an advance in the art of adsorbing latex polymers to find a way to make an adsorbing vinyl acetate latex-phosphorus acid polymer from concomitant polymerization of vinyl acetate and the phosphorus acid monomer without the aforementioned shortcomings.